feat: learnings system — git-backed markdown storage with agent tools, CLI, and HTTP serving

[priyanshujain]

Summary

Implements a learnings system that lets users save, read, search, and extract learnings as human-friendly markdown files backed by git.

Closes #105

• Core storage (service/learnings/): git-backed markdown store with Save, Read, List, Search, and Slug operations. Each save auto-commits to a local git repo at ~/.obk/learnings/.
• Agent tools (agent/tools/learnings.go): 4 tools — learnings_save, learnings_read, learnings_search, learnings_extract. Extract runs in a background goroutine with its own agent to parse source material.
• HTTP serving (GET /learnings/{topic}): serves markdown files via the existing server, no auth (behind ngrok).
• CLI (obk learnings open|list|search): browse and search learnings from the terminal.
• Wiring: registered in Telegram session (with push notifications + extract), CLI chat (save/read/search only), and system prompt.

New files (8)

File	Purpose
service/learnings/store.go	Git-backed markdown storage
service/learnings/store_test.go	10 store unit tests
agent/tools/learnings.go	4 tools + deps + notifier interface
agent/tools/learnings_test.go	14 tool unit tests
internal/server/handler_learnings.go	HTTP handler
internal/cli/learnings/learnings.go	CLI root + open + list
internal/cli/learnings/search.go	CLI search subcommand
spectest/learnings_test.go	3 spec tests (real LLM)

Modified files (6)

File	Change
config/paths.go	LearningsDir()
agent/tools/prompt.go	Learnings section gated on reg.Has("learnings_save")
internal/server/routes.go	GET /learnings/{topic}
internal/cli/root.go	Register learnings CLI command
channel/telegram/session.go	registerLearningsTools() with Telegram push notifications
internal/cli/chat.go	registerLearningsTools() (no notifier)

Test plan

• Unit tests: go test ./service/learnings/ ./agent/tools/ — 24 tests pass
• Spec tests: OBK_TEST_PROVIDER=gemini go test ./spectest/ -run TestSpec_Learning -v — 3 tests pass (SaveAndRead, SaveAndSearch, ListTopics)
• Full build: go build ./... — clean
• CLI smoke test: obk learnings list, save something via chat, obk learnings list again
• HTTP test: start server, curl http://localhost:8443/learnings/<topic>
• Telegram integration: say "save a learning about X", verify file + git commit + notification with link